The long-term objective of this project is to provide information that will be useful in assessing the risks posed to human health by the neurotoxic organophosphorus insecticides. It seeks to accomplish this objective by investigating preliminary observations that suggest the current view of how these toxic insecticides inhibit acetylcholinesterase is inadequate to describe the insecticide-enzyme interactions over a wide range of insecticide concentrations. Preliminary data indicates that the capacity of certain organophosphates to inhibit acetylcholinesterase changes as a function of inhibitor concentration. Moreover, evidence presented indicates that the organophosphate paraoxon causes a transient activation of acetylcholinesterase prior to inhibition. Collectively, these data suggest the presence of a secondary binding site on human recombinant acetylcholinesterase for certain organophosphates. Therefore, the overall hypothesis of this application is that organophosphates and their parent insecticides bind to a site on human recombinant acetylcholinesterase distinct from the catalytic triad, and that occupation of this site alters events at the active site. As a consequence of such binding, low exposures to certain organophosphorus insecticides could pose a greater health risk than is currently estimated based on dose-response relationships using higher exposure levels. The specific aims of this application are as follows: 1) To determine the bimolecular inhibition rate constant &,, for the inhibition of human recombinant acetylcholinesterase by the test organophosphates over a wide range of inhibitor concentrations; 2) To determine if the test organophosphates and their parent compounds can transiently enhance the hydrolysis of the substrate acetylthiocholine by human recombinant acetylcholinesterase prior to inhibition of the enzyme through phosphorylation; 3) To determine if the test organophosphates and their corresponding parent insecticides bind to the peripheral anionic site on human recombinant acetylcholinesterase; and 4) To determine if the rate of dephosphorylation of organophosphate- inhibited human recombinant acetylcholinesterase (reactivation) is altered by the occupation of the peripheral anionic site or some other secondary site for organophosphates.